1. Field of the Invention
The deposition of silica and various silicate salts in a variety of aqueous systems has long been a problem. These systems include, e.g., boilers and cooling towers. For the latter, the efficiency of the cooling system is often directly limited by high silica content of the water in many regions of the world, especially the western United States and Mexico, where surface and well waters have silica levels as high as 100 mg/L. It has recently been discovered that such silica and silicate salt deposits can also become a substantial problem in cooling towers in which the recirculating water has gone through a number of cycles, i.e., has been "cycled up". The formation of silica/silicate deposits may actually limit the number of such cycles to 2 to 3, thus imposing unsatisfactory restraints in areas where limited water supplies are a severe problem, or where waste treatment of large volumes of water has drawbacks. Also, in such cycled up cooling towers, the water is characterized by high pH and high calcite concentration. In aqueous systems having such severe conditions, not only are conventional agents for the control of calcium carbonate scale frequently unsuccessful in controlling such calcite scale deposits, but in a similar fashion, conventional agents for the control of silica and silicate salt deposits have no predictable or expected utility.
Silica (silicon dioxide) appears naturally in a number of crystalline and amorphous forms, all of which are water insoluble, thus leading to the formation of intractable deposits. Silicates are salts derived from silica or the silicic acids, especially orthosilicates and metasilicates, which may combine to form polysilicates. All of these, except the alkali silicates, are water insoluble. Magnesium silicate in particular is prevalent and has limited water solubility. A number of different forms of silica and silicate salt deposits are possible, and these will depend, among other factors, on temperature, pH and the concentration/ratio of calcium and magnesium in the make-up water.
Traditionally, deposition has been controlled by softening the makeup water to the system being treated, by blowdown, or by both. If deposition does occur, however, because the silica scale is so tenacious and hard to remove, it will often be necessary to use both mechanical removal and washing with ammonium fluoride or hydrofluoric acid in order to effectively remove the scale deposits. The use of agents for the control of silica and silicate deposition, such as those of the present invention, provide the advantage of avoiding such mechanical and chemical cleaning, which causes down time and increased energy and labor costs.
The phosphonate combinations of the present invention are useful for controlling the deposition of silica and silicates under severe conditions which include elevated pH; and so it should be noted that pH affects the ionization of silanol groups and, therefore, affects the polymerization rate. Silica first forms, then three-dimensional networks form. Eventually, colloidal particles grow through condensation. At pH 7, nuclei formation and particle growth is very rapid. The pH of cycled up cooling towers is usually at least 8.5 or higher.
Polyether polyamino methylene phosphonates, when used in combination with hydroxyphosphonoacetic acid (HPA) or amino tris methylene phosphonate (AMP), and in accordance with the method of the present invention, can prevent the deposition of silica and silicate salts. This method is especially useful under conditions of high pH and high calcite concentrations, e.g., those found in cycled up cooling towers. Various industrial and commercial water-carrying systems are subject to silica and silicate deposit formation problems. These deposits form frequently in the tubes of heat exchangers and on other heat exchange surfaces, such as those in cooling towers. Particular systems or applications areas where severe conditions, especially high alkalinity, lead to buildup of silica and silicate deposits, in addition to cycled up cooling towers, include reverse osmosis systems, sugar refining evaporators, and certain types of gas scrubbers.
The phosphonate combinations used in the methods of the present invention, are usually used in threshold inhibitor amounts. The compositions of the present invention, especially those in which the phosphonate combination is further combined with a polymer, have dispersant properties as well and significantly reduce the adherency of any deposits which are formed, facilitating their easy removal.
Particular problems are encountered when attempting to prevent deposits of silica and silicate salts under severe conditions, where conventional treatments do not provide complete control. Conventional treatment can be used to inhibit silica/silicate deposits under normal conditions of alkalinity, e.g., up to 100 to 120 times calcite saturation, i.e., a water containing Ca.sup.2+ and CO.sub.3.sup.2- present at 100 times (100.times.) their solubility limit of calcium as calcite (calcite is the most common crystalline form of calcium carbonate). However, what is desired are inhibitors effective in greater than 150.times. water, especially in greater than 250.times. water, and more especially in greater than 300.times. water, i.e., where the calcite ions can be prevented from precipitating as calcium carbonate scale using substoichiometric amounts of an inhibitor. The polyether phosphonate compositions used in the methods of the present invention are especially useful under severe conditions characterized by a calcite saturation level of 150.times. and above, especially 250.times. and above, and more especially 300.times. and above, as defined in the paragraph immediately below.
Another characteristic feature of the severe conditions under which the phosphonate combination compositions used in the methods of the present invention are especially useful is high pH, i.e. a pH of 8.5 and higher, particularly a pH of 9 or 10 or even higher.
One of the particular advantages of the polyether phosphonates used in the combination compositions used in the methods of the present invention is the exceptional calcium tolerance which they exhibit. Calcium tolerance is a measure of a chemical compound's ability to remain soluble in the presence of calcium ions (Ca.sup.2+). As pH increases, calcium tolerance decreases rapidly for many compounds which might be used to control silica/silicate deposits, and they precipitate with calcium at alkaline pH's, rendering them useless. While it is common practice to use an acid feed to the water of, e.g., a cooling tower system in order to lower pH and thus avoid the calcium tolerance problem, the danger to handlers which such acid feeding poses makes it all the more important to find inhibitors of silica/silicate deposits which operate at high pH's.
2. Brief Description of the Prior Art
It is known to use cationic polymers or cationic surfactants as silica scale inhibitors in hypersaline geothermal brines (Harrar, J. E. et al., "Final Report on Tests of Proprietary Chemical Additives as Anti-scalants for Hypersaline Geothermal Brine", January 1980, Lawrence Livermore Laboratory, Harrar, J. E. et al., "On-Line Tests of Organic Additives for the Inhibition of the Precipitation of Silica from Hypersaline Geothermal Brine IV, Final Tests of Candidate Additives", February 1980. Lawrence Livermore Laboratories; and Harrar, J. E. et al., "Studies of Scale Formation and Scale Inhibitors at the Salton Sea Geothermal Field", Corrosion/80. Paper No. 225, International Corrosion Forum, devoted exclusively to the Protection and Performance of Materials, Mar. 3-7, 1980. Chicago, Ill.).
U.S. Pat. No. 4,933,090 discloses the use of phosphonates such as hexamethylene diamine tetra (methylene phosphonic acid) and diethylene triamine penta (methylene phosphonic acid) and anionic polymers to control silica deposition.
U.S. Pat. No. 3,928,196 discloses the use of copolymers of 2-acrylamido-2-methylpropysulfonic acid and acrylic acid as scale inhibitors.
U.S. Pat. No. 4,640,793 discloses the use of admixtures containing carboxylic acid/sulfonic acid polymers and phosphonates as scale and corrosion inhibitors.
U.S. Pat. No. 4,618,448 discloses the use of polymers comprising an unsaturated carboxylic acid, an unsaturated sulfonic acid and an unsaturated polyalkylene oxide as scale inhibitors.
Japanese No. 57-084794 discloses the use of copolymers of acrylic acid and allyl polyethylene glycol as scale inhibitors.
U.S. Pat. No. 4,510,059 discloses the use of carboxylic functional polyampholytes to reduce silica deposits in aqueous systems.
U.S. Pat. No. 4,432,879 discloses the use of 2-phosphonobutane-1,2,4-tricarboxylic acid and methacrylic acid/2-acrylamido-2-methylpropyl sulfonic acid polymers to disperse solid matter such as clay including China Clay (Al.sub.2 O.sub.3.2H.sub.2 O.2SiO.sub.2) in aqueous systems. Threshold inhibition of silica/silicates is not disclosed or suggested.
U.S. Pat. No. 4,532,047 discloses a method of inhibiting amorphous silica scale formation using polypolar organic compounds and borate ion sources.
U.S. Pat. No. 4,584,104 discloses a method of inhibiting amorphous silica scale formation using a source of orthoborate ions.
U.S. Pat. No. 4,080,375 discloses methylene phosphonates of amino-terminated oxyalkylates for use as scale inhibitors, but these compositions are not the same as those used in the method of the present invention, nor is there any suggestion that such compositions would be useful for inhibiting the deposition of silica/silicates.
U.S. Pat. No. 4,931,189 discloses aminomethylene phosphonates of the type used in the method of the present invention, but for inhibiting oil field scale formation involving a high brine environment susceptible to gypsum or barite scale formation. Such use in no way suggests the inhibition of silica/silicate scale deposition described herein.
The polyether polyamino methylene phosphonates of the type which are used to inhibit silica/silicate scale deposition in the method of the present invention, are described in copending application Ser. No. 07/879,231, filed May 11, 1992. While their use for the control of calcium carbonate scale under severe conditions which include elevated pH and high calcium carbonate saturation levels, is described, and their combination with hydroxyphosphonoacetic acid for preventing steel corrosion is also disclosed, there is no suggestion of their use or the use of the combination to inhibit silica/silicate scale deposition.
U.S. Pat. No. 4,874,527 discloses a method for controlling the formation of silica/silicate deposits in aqueous systems by adding an imine polymer, a phosphonate and, optionally, a source of molybdate or borate ions to the aqueous system.
U.S. Pat. No. 5,078,879 discloses a method for controlling the formation of silica/silicate deposits by adding 2-phosphonobutane-1,2,4-tricarboxylic acid and, optionally, an anionic polymer.